In my experience, Perl has inherent problems with readability on a line-by-line basis. I don't think that there's anything that can be added to change this - testing would prove a line does certain things but given that a test isn't complete specification, testing couldn't by itself change the readability problem.
Again, Perl would have more justification if Ruby wasn't visible
If someone references a book with author/title, PLEASE at least check a review on the web before commenting.
I was hoping that any answer might be insightful.
(Readability is mostly a function of familiarity. Everything has advantages/disadvantages, if I'd list the problems with Perl, readability wouldn't be there.)
Again, Perl would have more justification if Ruby wasn't visible